The invention relates to a method for manufacturing a panel made of composite material incorporating a lightning protection means and also to a panel made of composite material manufactured by way of the method.
As is known, the fuselage of an aircraft comprises a juxtaposition of metal sheets or panels. In the event of a lightning strike, this metal fuselage forms, in the manner of a Faraday cage, a means of protection for the elements disposed inside (in particular the electrical systems which are connected to the fuselage of the aircraft) and allows the electric current to flow from the front tip to the rear tip of the fuselage.
In order to reduce the mass of the aircraft, the fuselage panels are made of composite material, in particular composite material reinforced with carbon fibers of the CFRP or CRP type (for carbon fiber reinforced polymer or carbon fiber reinforced plastic).
These panels made of composite material are obtained by laying on a mold plies of carbon fibers generally preimpregnated on a support. Once all of the plies have been laid, the panel is then subjected to a consolidation or polymerization step.
In addition to saving mass, this type of composite material has good mechanical properties, an absence of corrosion and good fatigue behavior.
However, in contrast to metal panels made of aluminum alloy, these panels made of composite material have low electrical conductivity.
Consequently, it is necessary to add to these panels made of composite material a lightning protection means known as LSP (for lightning strike protection).
This lightning protection means is generally fitted prior to the consolidation or polymerization, either directly on the mold prior to the laying of the plies of fibers, or after the laying of the plies of fibers.
As illustrated in FIG. 1B, a fuselage panel 10 comprises a first radius of curvature in a first direction X and a second radius of curvature in a second direction Y.
In the rest of the description, the direction X corresponds to the longitudinal direction of the aircraft (which runs from front to rear) and the direction Y corresponds to a transverse direction, perpendicular to the longitudinal direction X.
This panel 10 made of composite material comprises a lightning protection means 12.
According to one embodiment, a lightning protection means comprises a conductive layer (a mesh, a conductive epoxy coating or a metal foil), a support layer and a resin film.
According to one procedure, the plies of carbon fibers are laid in position at a first station. Next, the lightning protection means is fitted at a second station by manually juxtaposing lightning protection strips 14. These strips are disposed in one direction and have edges which overlap in order to ensure electrical continuity.
The strips 14 are cut from rolls and have a width which can reach 900 mm.
This procedure makes it possible to control the placing of the strips on a surface comprising a radius of curvature, or two radii of curvature, such as an aircraft fuselage panel.
In order to increase productivity, one solution consists in automating the laying of the lightning protection strips.
A first solution consists in using an automatic draping machine of the ATL type (for automated tape laying). This type of machine makes it possible to lay strips with a width of between 150 and 300 mm. This type of machine makes it possible to ensure that the edges of the strips overlap in order to ensure electrical continuity. However, it can only be used to lay strips on a flat support, this not being the case in a fuselage panel. Even if it were conceivable to lay the plies of fibers and the lightning protection strips flat and then to deform the assembly in order to obtain a curved panel, the electrical continuity between the strips could no longer be ensured after deformation.
A second solution consists in using a fiber placement machine of the AFP type (for automated fiber placement). This type of machine makes it possible to lay strips on curved surfaces such as a fuselage panel. However, this type of machine only makes it possible to lay strips with a width that does not exceed 2 inches, or around 5 cm.
As illustrated in FIG. 2, the lightning protection means comprises strips 16 laid on the fuselage panel 10 which are mutually parallel and arranged in the direction X. These strips 16 have a width that does not exceed 2 inches in order to be able to be placed on a curved surface in an automated manner. In this case, it is found that the electrical conductivity in the direction X is different from the electrical conductivity in the direction Y (with a ratio of 10 when the conductivity is expressed in mOhm per unit area). The size of the difference between these two values reduces the efficiency of the lightning protection means. In addition, it does not ensure the protection of the electrical systems present in an aircraft of which the fuselage is produced in this way.